In the fields where pigments or dyes insoluble in solvents are used with dispersing them in liquid media, the particle size of such pigments or dyes are made very small and various measures are proposed to improve the storage stability of dispersions. For example, UK 2 001 083 discloses the use of a specific polyester amine as a dispersant for pigments in paints or inks. JP-A-9-87570 discloses an aqueous pigment dispersion comprising a water-soluble nitrogen-containing acrylic resin which is prepared by copolymerizing an α,β-ethylenically unsaturated carboxylic acid and an α,β-ethylenically unsaturated nitrogen-containing monomer. JP-A-2000-242486 discloses an aqueous ink composition for ink jet printing comprising a general acrylic acid copolymer as a dispersant.
JP-A-11-269418 discloses an aqueous ink composition for ink jet printing comprising a block copolymer, and JP-A-10-87768 discloses an aqueous ink composition for ink jet printing comprising a graft-polymer.
JP-A-11-080633 discloses an ink for ink jet printing containing coloring resin particles which consist of pigment particles coated (micro-capsulated) with a resin having a carboxyl group.
However, the kinds of pigments, which can be used in dispersion compositions comprising general dispersion stabilizers or the above pigment dispersants are limited. Thus, in fields which require full color printing such as printing inks or ink-jet printing inks, ink compositions have been developed using various dispersants suitable for cyan, magenta, yellow and black colorants respectively. Thus, when ink compositions are formulated, it is difficult for all the four color inks have the same composition, and each ink composition should be specifically formulated, which takes labors and times. Furthermore, only a few ink-jet printing inks have satisfactory ejection stability.
When the block copolymer, the graft polymer or the micro-capsulated pigments are used, it may be possible to formulate the same composition for all the colors. However, the synthesis of such polymers and the production of microcapsules require various production steps and thus need increased costs and energy for producing dispersion compositions in comparison with the synthesis of usual copolymers.